git-svn: fix blocking with svn:// servers after do_switch

We now explicitly disconnect before starting new SVN::Ra
connections.  SVN::Ra objects will automatically be disconnected
from the server on DESTROY.

SVN servers seem to have problems accepting multiple connections
from one client, and the SVN library has trouble being connected
to multiple servers at once.  This appears to cause opening the
second connection to block, and cause git-svn to be unusable
after using the do_switch() function.

git-svn opens another connection because a workaround is
necesary for the buggy reparent function handling on certain
versions of svn:// and svn+ssh:// servers.  Instead of using the
reparent function (analogous to chdir), it will reopen a new
connection to a different URL on the SVN server.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-svn.perl

@@ -3002,6 +3002,7 @@ sub new {
 	      \&Git::SVN::Prompt::username, 2),
 	  ]);
 	my $config = SVN::Core::config_get_config($config_dir);
+	$RA = undef;
 	my $self = SVN::Ra->new(url => $url, auth => $baton,
 	                      config => $config,
 			      pool => SVN::Pool->new,